This is going to take a while. Eiyuu Senki so far has three games to its name, World Conquest, Gold and the WWX game. World conquest is the first of the games and ironically has the most plot compared to the other two (respective to those games at least) but, seeing as its the first game, there are fewer girls to use and most of the individual events in the game center around Himiko, the heroes from Zipang and Arthur, and somewhat important, the mechanics and features were still kind of lacking compared to Gold. Gold (this game) introduces new features, such as the item synthesizing/shop and after battle achievement rewards and New Game Plus achievement system, increased the roster of girls for your use and has managed to even out individual character events to all of the girls respectively. Of course there are two major downsides of Gold as well. First, the plot of Gold is significantly reduced to something more generic in plotlines compared to the original and the said individual events of the girls tend to fall into roughly the same type of plot as well, more or less. Due to this, GOLD, for most people, tend to see it more as a fandisc that mainly caters to having more girls to romance and do the thing with than the original at the expense of the plot.
The third game, WWX, is a gacha game that is previously Japan-only, with announcements of an English release just appeared recently, so I can't really say much about the plot, but from what I've heard around SadPanda and some comments here, it's basically what you'd expect from a gacha game. Sure there are more girls than both Gold and the Original Combined (just and expression), but it's a gacha game. You do the math there.